get coordinates of bone from the perspective of second bone in driver
Although not in the same direction as i was originally thinking, one can get a good sense of which direction a bone is at using the following construction:

create a helper bone parented to and aligned with bone A, with a damped track constraint pointing to bone B. Now add the driver using the desired rotation of the helper bone  (e.g., X, Z) in swing and Y twist mode within local space. In this mode the angle is calculated as if the bone was first swung in the X,Z axis to align the bone, and then twisted in Y to match up the axis. This thus gives us an indication of the direction of bone B from the perspective of bone A without having to worry about any twisting around the Y axis of our helper bone.